Thanks Kathy! Wow, I thought that it was just we poultry hobbyists that were confused and/or misinformed about the treatment of blackhead. But It looks there is some confusion and misinformation in the Veterinary community as well!
Here is a list of substances that were found to be effective (and ineffective) for treating blackhead in the research study that I referenced above: "Dimetridazole, metronidazole, ornidazole, and tinidazole were highly effective at 200 ppm in feed. Paromomycin sulfate, and carbadox were ineffective in vivo, with no improvement in liver or cecal lesion scores compared to that of infected controls."
Of these known effective treatments, dimetridazole has been banned for use with poultry in the US. This is not related to its effectiveness as a treatment for blackhead in poultry, but because of its carcinogenic and mutagenic effects on humans who consume treated poultry! Apparently this was once widely used in poultry food for commercial growers in the US. But the banning of this once widely used treatment may be what has confused some vets.
Metronidazole is widely used to treat various digestive disorders in dogs and cats, and should be readily available from a small animal veterinarian (or OTC for aquarium fish). The tablet form and dosage is not terribly convenient for administering to poultry, but consider the alternative (of not having an effective treatment for blackhead).
Ornidazole, and tinidazole are used to treat protozoan infections in humans and so would require a prescription from an MD. And so, as Kathy has been emphasizing, metronidazole is our best option for treating poultry with blackhead.
But note that the -idazole terminology suggests that all these effective antiprotozoan compounds have a common chemical base, so the concerns raised about human consumption of dimetridazole-treated poultry probably applies to these other compounds as well. So OK for pets but not for animals in the food chain.